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<span class="breadcrumb-node">词干提取算法</span>
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<div class="titlepage"><div><div>
<h2 class="title">
<a id="algorithmic-stemmers"></a>词干提取算法<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/230_Stemming/10_Algorithmic_stemmers.asciidoc">edit</a>
</h2>
</div></div></div>
<p>Elasticsearch 中的大部分 stemmers （词干提取器）是基于算法的，它们提供了一系列规则用于将一个词提取为它的词根形式，例如剥离复数词末尾的 <code class="literal">s</code> 或 <code class="literal">es</code> 。提取单词词干时并不需要知道该词的任何信息。</p>
<p>这些基于算法的 stemmers 优点是：可以作为插件使用，速度快，占用内存少，有规律的单词处理效果好。缺点是：没规律的单词例如 <code class="literal">be</code> 、 <code class="literal">are</code> 、和 <code class="literal">am</code> ，或 <code class="literal">mice</code> 和 <code class="literal">mouse</code> 效果不好。</p>
<p>最早的一个基于算法的英文词干提取器是 Porter stemmer ，该英文词干提取器现在依然推荐使用。 Martin Porter 后来为了开发词干提取算法创建了 <a href="http://snowball.tartarus.org/" class="ulink" target="_top">Snowball language</a> 网站， 很多 Elasticsearch 中使用的词干提取器就是用 Snowball 语言写的。</p>
<div class="tip admon">
<div class="icon"></div>
<div class="admon_content">
<p><a href="/guide/en/elasticsearch/reference/5.6/analysis-kstem-tokenfilter.html" class="ulink" target="_top"><code class="literal">kstem</code> token filter</a> 是一款合并了词干提取算法和内置词典的英语分词过滤器。为了避免模糊词不正确提取，这个词典包含一系列根词单词和特例单词。 <code class="literal">kstem</code> 分词过滤器相较于 Porter 词干提取器而言不那么激进。</p>
</div>
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<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="_使用基于算法的词干提取器"></a>使用基于算法的词干提取器<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/230_Stemming/10_Algorithmic_stemmers.asciidoc">edit</a>
</h3>
</div></div></div>
<p>你可以使用 ​<a href="/guide/en/elasticsearch/reference/5.6/analysis-porterstem-tokenfilter.html" class="ulink" target="_top"><code class="literal">porter_stem</code></a>​ 词干提取器或直接使用 <a href="/guide/en/elasticsearch/reference/5.6/analysis-kstem-tokenfilter.html" class="ulink" target="_top"><code class="literal">kstem</code></a> 分词过滤器，或使用 <a href="/guide/en/elasticsearch/reference/5.6/analysis-snowball-tokenfilter.html" class="ulink" target="_top"><code class="literal">snowball</code></a> 分词过滤器创建一个具体语言的 Snowball 词干提取器。所有基于算法的词干提取器都暴露了用来接受 <code class="literal">语言</code> 参数的统一接口： <a href="/guide/en/elasticsearch/reference/5.6/analysis-stemmer-tokenfilter.html" class="ulink" target="_top"><code class="literal">stemmer</code> token filter</a> 。</p>
<p>例如，假设你发现 <code class="literal">英语</code> 分析器使用的默认词干提取器太激进并且你想使它不那么激进。首先应在 <a href="/guide/en/elasticsearch/reference/5.6/analysis-lang-analyzer.html" class="ulink" target="_top">language analyzers</a> 查看 <code class="literal">英语</code> 分析器配置文件，配置文件展示如下：</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">{
  "settings": {
    "analysis": {
      "filter": {
        "english_stop": {
          "type":       "stop",
          "stopwords":  "_english_"
        },
        "english_keywords": {
          "type":       "keyword_marker", <a id="CO154-1"></a><i class="conum" data-value="1"></i>
          "keywords":   []
        },
        "english_stemmer": {
          "type":       "stemmer",
          "language":   "english" <a id="CO154-2"></a><i class="conum" data-value="2"></i>
        },
        "english_possessive_stemmer": {
          "type":       "stemmer",
          "language":   "possessive_english" <a id="CO154-3"></a><i class="conum" data-value="2"></i>
        }
      },
      "analyzer": {
        "english": {
          "tokenizer":  "standard",
          "filter": [
            "english_possessive_stemmer",
            "lowercase",
            "english_stop",
            "english_keywords",
            "english_stemmer"
          ]
        }
      }
    }
  }
}</pre>
</div>
<div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td align="left" valign="top" width="5%">
<p><a href="#CO154-1"><i class="conum" data-value="1"></i></a></p>
</td>
<td align="left" valign="top">
<p><code class="literal">keyword_marker</code> 分词过滤器列出那些不用被词干提取的单词。这个过滤器默认情况下是一个空的列表。</p>
</td>
</tr>
<tr>
<td align="left" valign="top" width="5%">
<p><a href="#CO154-2"><i class="conum" data-value="2"></i></a><a href="#CO154-3"></a></p>
</td>
<td align="left" valign="top">
<p><code class="literal">english</code> 分析器使用了两个词干提取器： <code class="literal">possessive_english</code> 词干提取器和 <code class="literal">english</code> 词干提取器。   所有格词干提取器会在任何词传递到  <code class="literal">english_stop</code> 、 <code class="literal">english_keywords</code> 和 <code class="literal">english_stemmer</code> 之前去除 <code class="literal">'s</code> 。</p>
</td>
</tr>
</table>
</div>
<p>重新审视下现在的配置，添加上以下修改，我们可以把这份配置当作新分析器的基本配置：</p>
<div class="ulist itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
修改 <code class="literal">english_stemmer</code> ，将  <code class="literal">english</code> （​<a href="/guide/en/elasticsearch/reference/5.6/analysis-porterstem-tokenfilter.html" class="ulink" target="_top"><code class="literal">porter_stem</code></a>​ 分词过滤器的映射）替换为 <code class="literal">light_english</code> （非激进的 <a href="/guide/en/elasticsearch/reference/5.6/analysis-kstem-tokenfilter.html" class="ulink" target="_top"><code class="literal">kstem</code></a> 分词过滤器的映射）。
</li>
<li class="listitem">
添加 <a class="xref" href="asciifolding-token-filter.html" title="如果有口音"><code class="literal">asciifolding</code></a> 分词过滤器用以移除外语的附加符号。
</li>
<li class="listitem">
移除 <code class="literal">keyword_marker</code> 分词过滤器，因为我们不需要它。（我们会在 <a class="xref" href="controlling-stemming.html" title="控制词干提取">控制词干提取</a> 中详细讨论它）
</li>
</ul>
</div>
<p>新定义的分析器会像下面这样:</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">PUT /my_index
{
  "settings": {
    "analysis": {
      "filter": {
        "english_stop": {
          "type":       "stop",
          "stopwords":  "_english_"
        },
        "light_english_stemmer": {
          "type":       "stemmer",
          "language":   "light_english" <a id="CO155-1"></a><i class="conum" data-value="1"></i>
        },
        "english_possessive_stemmer": {
          "type":       "stemmer",
          "language":   "possessive_english"
        }
      },
      "analyzer": {
        "english": {
          "tokenizer":  "standard",
          "filter": [
            "english_possessive_stemmer",
            "lowercase",
            "english_stop",
            "light_english_stemmer", <a id="CO155-2"></a><i class="conum" data-value="1"></i>
            "asciifolding" <a id="CO155-3"></a><i class="conum" data-value="2"></i>
          ]
        }
      }
    }
  }
}</pre>
</div>
<div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td align="left" valign="top" width="5%">
<p><a href="#CO155-1"><i class="conum" data-value="1"></i></a><a href="#CO155-2"></a></p>
</td>
<td align="left" valign="top">
<p>将 <code class="literal">english</code> 词干提取器替换为非激进的 <code class="literal">light_english</code> 词干提取器</p>
</td>
</tr>
<tr>
<td align="left" valign="top" width="5%">
<p><a href="#CO155-3"><i class="conum" data-value="2"></i></a></p>
</td>
<td align="left" valign="top">
<p>添加 <code class="literal">asciifolding</code> 分词过滤器</p>
</td>
</tr>
</table>
</div>
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